Since the X-ray is discovered, the X-ray radiography technology has been widely used as a non-destructive inspection measure in the medical, security, anti-smuggling, and many other fields. The development of the X-ray radiography technology is begun with the initial X-ray film photography to CR (Computed Radiography), then to DR (Digital Radiography), to dual-energy spectrum DR imaging, and even to multi-energy spectrum DR imaging technology. The basic principle of the X-ray radiography technology may be described as follows. An X-ray is emitted to an object to be inspected and interacted with the material of the object. The X-ray attenuated after passing through the object to be inspected is received by a detector and converted into an electronic signal to form an image. Magnitude of signal strength of each pixel in the image reflects the degree of absorption of the X-photons by the material in the direction of the X-ray transmission for this pixel, that is, the integral of the X-ray linear attenuation coefficients for all materials in the direction of the X-ray transmission path. Therefore, a mono-energy spectrum radiography cannot directly provide material information, such as density of the object to be inspected or atomic number, and the shape information of each object in the image is usually required for determining the object to be inspected. However, the shape information is often inaccurate, especially for materials without a fixed shape, such as explosives, drugs, gasoline, etc., resulting in a false positive or a false negative.
Due to the above limit in the mono-energy spectrum radiography, the need of accurate recognition of the object material cannot be met and the dual-energy radiography technology is proposed. Unlike the mono-energy spectrum radiography, a dual-energy radiography system uses two sets of radiographic images with different energy spectrums and uses a dedicated algorithm for material recognition which may achieve recognition of specificity of different substances or materials. Such a recognition cannot be achieved by the mono-energy spectrum radiography technology.
The interactions between the X-ray photons and the substance are typically divided into three categories: the photoelectric effect, the Compton scattering, and the electron pair effect, and the reaction cross-sections for the three categories of interactions are related to the X photon energy and atomic number of the substance. For a given substance, its X-ray linear attenuation coefficient is equal to the product of the total atomic cross-section and the atomic density. Therefore, by measuring the X-ray linear attenuation coefficient of the substance under different energy spectrums, the atomic number of the substance and the electron density can be determined by using the relation between the X-ray linear attenuation coefficient and the total atomic cross-section, and the recognition of the material is thus achieved.
The current dual-energy radiographic system typically uses a dual-energy curve based method for material recognition to calculate the atomic number quickly and accurately, and a qualitative and quantitative evaluation of the material is achieved based thereon. Based on different application requirements, the existing dual-energy radiography technology may be divided into two different categories of low-energy dual-energy and high-energy dual-energy:
1) the low-energy dual-energy typically refers to a case where the maximum X-ray photon energy <=450 keV where the interactions between the X-ray photons and the substance have only two categories: the photoelectric effect and the Compton scattering. In the low energy region, the photoelectric effect is dominant and related to Z intensity; and in the medium low region, the Compton scattering is dominant and basically not related to Z. The ratio of the attenuation coefficients under the two energies is changed monotonically with Z, resulting in a better discrimination. The material recognition can be achieved based on this ratio. The low-energy dual-energy radiography technology is typically applied to cases where the object has a smaller volume and a rather lower density, such as luggage item machine, absorptiometry, coal ash analyzer, etc.
2) the high-energy dual-energy typically refers to a case where the maximum X-ray photon energy >=1 MeV where the interactions between the high-energy X-ray photons and the substance are different from those for the low-energy photons: the three categories of interactions, i.e. the photoelectric effect, the Compton scattering and the electron pair effect, coexist. Therefore, their methods for implementing material recognition and their capabilities of recognition are different. In general, in the MeV energy region, due to a small gap between high-low energy curves for different substances, its material recognition capability is weaker than that for low-energy dual-energy case. The high-energy radiography is typically applied to inspection of material having a high atomic number which cannot be penetrated by the low-energy X-ray or inspection of an object having a large volume, such as, inspection of radioactive materials, large container inspection system, the flight case inspection system, and non-destructive testing of large metal parts, or the like.
However, the above currently widely used dual-energy radiography technology still has its inherent drawbacks. Since in the radiography the X-ray transmissive scanning is performed on the object at a certain view angle only, the acquired high-low energy projection data is representative of integral information of all materials on each X-ray straight propagation path. Therefore, when there are multiple kinds of materials on this path, the dual-energy radiography cannot distinguish such an overlapping relation of materials located front and rear. In other words, the dual-energy radiography can only recognize a single material (pure substance) correctly without any overlapping relation. For cases where two or more kinds of materials are presented in the direction of X-ray transmission with an overlapping relation, the existing dual-energy DR imaging technology cannot distinguish them and can only recognize it as a new mixed material, resulting in an inaccurate recognition of the original materials and therefore an error in recognition. This drawback also imposes a limit to the value of the dual-energy X-ray DR imaging technology in the field of security inspection.
To solve the problem of correctly recognizing the overlapped materials, the prior art typically tries to avoid the overlapped areas as much as possible by using multiple additional view angles and capturing the dual-energy radiographic images at different angles; however, this method cannot guarantee that the problem can be solved completely. Another solution is to use a more complicated CT (computed tomography) technology to capture radiographic images at hundreds or thousands of angles throughout a range of 360°. For example, a dedicated CT reconstruction algorithm is used for direct calculation to acquire a full 3D image information of the object, and the problem of overlapped substance is solved fundamentally. However, both of the hardware of the imaging system in the above two solutions are to be changed greatly, especially for the CT technology. This will greatly increase the hardware cost and technical problem of the dual-energy imaging system, and also increase the difficulty in maintenance of the system and reduce its application scope.